‘Is It Hot In Here?’ Human Body Temperature Decreased Over Last 200 Years

It has long been believed that 98.6°F is the normal human body temperature, but recent evidence coming out of the Stanford University School of Medicine (SUSM) contends that might no longer be true. According to researchers at the SUSM, since the 1800’s, the average human body temperature in the United States has actually dropped almost a full degree.

“Our temperature’s not what people think it is,” said Julie Parsonnet, MD, and professor of medicine and of health research and policy at SUMS. “What everybody grew up learning, which is that our normal temperature is 98.6°F, is wrong.”

The 98.6°F that we have come to understand as the average human body temperature was calculated in 1851 by German physician, Carl Reinhold August Wunderlich. However, a recent survey of 25,000 British patients found the average body temperature to actually be 97.9°F. Similarly, a recent study published in eLife looking at 677,423 temperature measurements found that, “from three cohorts

• the Union Army Veterans of the Civil War (N = 23,710; measurement years 1860–1940),
• the National Health and Nutrition Examination Survey I (N = 15,301; 1971–1975),
• and the Stanford Translational Research Integrated Database Environment (N = 150,280; 2007–2017)

[researchers] determined that mean body temperature in men and women, after adjusting for age, height, weight and, in some models date and time of day, has decreased monotonically by 0.03°C per birth decade.” It also turns out that men and women are cooling at different rates as well. The body temperatures of men (who have a lower body temperature that women on average) born in the 2000s is on average 1.06°F lower than that of men born in the early 1800s, while the body temperature of women is on average 0.58°F lower.

But what is causing the decline in body temperature? Researchers posit that it “is the result of changes in our environment over the past 200 years, which have in turn driven physiological changes.” One contribution to our decreased body temperature could be explained by a reduction in metabolic rate. Parsonnet hypothesises that may be due to a population-wide decline in inflammation as a result of better medical care explaining that, “Inflammation produces all sorts of proteins and cytokines that rev up your metabolism and raise your temperature.”

Additionally, researchers hypothesise that modern day heating and cooling could have contributed to a lower metabolic rate. While homes in the 19^th century had irregular heating and no cooling, today, central heating and air conditioning are commonplace, thus removing the need for humans to expend energy to maintain a constant body temperature. 

Ultimately, the study sheds light on that deeper truth of constant physiological change in humans from generation to generation as a result of human evolution and technology. “Physiologically, we’re just different from what we were in the past,” Parsonnet said. “The environment that we’re living in has changed, including the temperature in our homes, our contact with microorganisms and the food that we have access to. All these things mean that although we think of human beings as if we’re monomorphic and have been the same for all of human evolution, we’re not the same. We’re actually changing physiologically.”